Prime Minister John Key has signalled Justice Minister Judith Collins is on her final warning after she withheld details of a dinner in China with a company linked to her husband.

The dinner was with senior members of Oravida, a company that deals with New Zealand dairy products, and a senior Chinese government official. Collins' husband David Wong-Tung is a director of the company.

Key met Collins this morning to express his disappointment that she did not come clean when questions were first raised about her visiting Oravida offices during her official visit to China last year.

Key said the cumulative effects of her interactions with Oravida could lead to a perception of a conflict of interest.

"I made clear to her in no uncertain terms how disappointed I was," he said.

"She's an outstanding minister but she's handled this situation very poorly and I've made my feelings known to her directly this morning."

Key said he had been left exposed because he had based his public defence of Collins on the incomplete information she had given him.

Collins had "misled by omission" and had a responsibility to reveal all the meetings she had in China, private or not, saying "she certainly should have made me aware of [the dinner]".

"Judith had a responsibility when she was asked the questions not just to answer the question directly but to tell me and the New Zealand public everything that had gone on," he said.

Instead, she had allowed events to accumulate, leading to the perception of a conflict of interest "and that's unacceptable".

Collins had assured him she did not discuss matters of national interest with the Chinese official and that it was just a personal dinner, and he accepted that, he said.

When asked whether this was a final warning for Collins he said "I wouldn't want to be in her shoes if there was a repeat of it".

"I just hope there wouldn't be a repeat of this," he said.

In a hastily arranged media conference this morning, Collins apologised for failing to divulge the details.

She was told to front up to the media after the prime minister's chief of staff, Wayne Eagleson, learnt last night about the dinner, apparently after Collins was summoned to go over details of her trip as controversy over dealings with Oravida widened.

Collins confirmed she was forced to give an explanation to Key this morning about the lapse but refused to confirm if she had apologised to him.

The dinner with Oravida chairman Stone Shi and managing director Julia Xu took place in Beijing when Collins was there on official government business.

Collins said this morning she had not thought to mention the dinner when questions were asked about her visit to Oravida last week because it had been a private affair.

Shi was a very close personal friend and they had spent Christmas together.

She confirmed that she met Oravida representatives on three occasions during her visit - at a lunch also attended by Fonterra, Nestle and Ministry of Foreign Affairs representatives, at the dinner in Beijing later that evening, and at the company's premises in Shanghai.